In a gas turbine engine, hot section components, such as combustor liners, transition ducts, turbine airfoils, and turbine rings, require cooling airflows which are typically drawn from the high-pressure compressor. This air bypasses the combustion chamber, reducing the efficiency of the machine. There is a long-standing need for cooling schemes that require the use of less cooling air for improved engine efficiency and emission control.
Impingement and convection cooling have been used on interior surfaces of component walls (backside cooling). Film cooling has been used on exterior surfaces exposed to the hot combustion gas to cool the boundary layer of hot gas flow. Exterior surfaces may have thermal barrier coatings (TBC) for thermal insulation. Component walls are typically solid except for film cooling passages through the walls. Component structural walls are mostly cooled through heat conduction through the wall, interior surface coolant impingement and convection, and exterior surface film cooling.
In transpiration cooling, a coolant such as air is forced through a porous wall. It provides convection cooling inside the wall and efficient film cooling via outlet pores on the hot exterior surface of the wall. Transpiration cooling of gas turbine component walls has been implemented in various ways that generally fall into two categories: 1) holes drilled through the wall or formed by casting or molding the wall around removable pins. 2) Fibrous or other randomly porous material, including partially sintered ceramic or metal, metal felt or foam, and ceramic felt or foam. Randomly porous materials are generally not be suitable for high temperature, high load-bearing structures because they have random anisotropies due to uneven distributions of voids and solids that can cause weak points and uneven cooling. In addition, random fibrous structures are not inherently geometrically rigid.